Microbial defense against a lethal fungal pathogen of amphibians: Characteristics of an effective bioaugmentation strategy

Background/Question/Methods

Amphibians are currently threatened by the rapid global spread of the pathogen, Batrachochytrium dendrobatidis (Bd), which is the largest disease threat to biodiversity. To mitigate the threat, probiotic therapy through bioaugmentation is a feasible strategy based on growing evidence that microbes contribute to host defenses of plants and animals.  Bioaugmentation of locally-occurring protective bacteria on amphibians has mitigated this disease effectively in laboratory trials and one recent field trial. Areas still naïve to Bd provide an opportunity to proactively implement probiotic strategies to prevent further amphibian declines. In areas where Bd is endemic, bioaugmentation can facilitate repatriation of susceptible amphibians currently maintained in assurance colonies. 

Results/Conclusions

To move forward with an effective probiotic strategy, characteristics of effective probiotics must be identified in relation to their interactions with Bd, their host, other resident microbes, and the environment.  Importantly, the potential probiotic must inhibit Bd over a range of environmentally relevant conditions. We developed sampling strategies and filtering protocols to identify probiotics that inhibit Bd under ecologically relevant conditions and that persist on at-risk species. Samples of skin microbes are collected on high-risk species and on a broad spectrum of host species in communities affected or likely to be affected by the pathogen.  It is determined which isolates inhibit Bd and which probiotic isolates will persist on target amphibian species and in the environment as a reservoir.  Next, randomized clinical trials are conducted first in the laboratory and then in the field to determine if the probiotic is effective at inhibiting Bd. Additional considerations may become important in probiotic selection.  As examples, recent work shows that amphibians’ defensive antimicrobial peptide secretions and microbial secretions interact synergistically to inhibit Bd, and bacterial species also can act synergistically to inhibit Bd.  A bioaugmentation approach must use locally-occurring microbes, and biosafety testing is necessary to ensure that the probiotic candidate does not have non-target effects in the ecosystem.  This filtering tool is being used proactively to guide amphibian disease mitigation in Madagascar, where 500 endemic frog species occur, and Bd is likely to arrive at any time.  Over 1000 individuals from 120 frog species have been sampled in replicate rainforest and central plateau sites in two seasons.  Thus far, 80 isolates have been challenged against Bd, and seven were inhibitory.  Other taxa threatened by emerging infectious diseases, including bats, may benefit from this filtering protocol.